<紀要論文>
Lead-Zinc Ore Deposits of the Tsushima Islands, Nagasaki Prefecture, with Special Reference to Shigekuma-type Mineralization

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概要 Lead-zinc ore deposits of the Tsushima Islands have been studied in terms of geology and mineralogy in order to reveal the essential factors contributed to localize the ore deposits, and also to estab...lish spacial and paragenetic relations of mineralization concerned. The mineral assemblage, the variation of chemical compositions of minerals, and the characteristics of zonality have been discussed, so that the formation mechanism of the Shigekuma-type mineralization in Kamishima (North Tsushima) is clarified in comparison with that of the Taishu-type one in Shimojima (South Tsushima). The geologic feature of the insular province is characterized with a thick pile of mudstone and sandstone of the Taishu Group, which deposited under the deltaic to shallow sea environments in late Oligocene to early Miocene. The group shows rather intensively folded structure and extensive fracturing as a result of a tectonic movement during the period of late Daijima to Nishikurosawa stages which acted in cooperation with both Mizuho and Takachiho orogenies. The Taishu Group was intruded by sills and dikes of plagiophyre, quartz porphyry, dolerite and rhyolite, and by bosses and cryptobatholiths of biotite granite, just before and after the tectonic movement. The series of ore mineralization was probably formed in the period of Onnagawa stage, middle Miocene, being closely related to the granite intrusion. All the ore deposits take the form of fissure-filling veins and are distributed in eight areas, namely, Sago, Nita-Miné, and Kin areas in Kamishima, and Ohfunakoshi, Izuhara, Sasu (Taishu mine), Tsutsu and Yora areas in Shimojima. They are impreganted in shear fractures along N-S and NE faults, and also in tension fractures along NW faults. The former fractures are well developed in the western part of Tsushima, where the folding is more intense, and the latters are dominant in the eastern part, especially around the axes of anticlinoria. The formation of fractures and the ore localization are controlled not only by lithofacies but also by the primary sedimentary structure. Although every ore deposit forms in the same kinds of fracture system, and is composed principally of lead-zinc-iron sulfide and carbonate minerals, ore deposits in Kamishima (Shigekuma-type) are distinct in some characters from those in Shimojima (Taishu-type). Characteristics of the Shigekuma-type deposits are: 1) The mineralization started with deposition of Zn-Fe stage, followed by Cu-Ag and Pb stages, and ended in Fe-carbonate stage. 2) Relations between the distribution of minerals in each stage and the inter-mineralization fracturing indicate that ore-foming solutions with different compositions ascended along fissures repeatedly, while fracturing was taken place under a serial stress condition. This mineralization corresponds to the stage by stage zonality, especially to the tectonic opening-type in its zonality in SMIRNOV's sense. 3) No composition relations of main constituent minerals are observed with depth in each area of mineralization stage. 4) There crop out no acidic igneous rocks around the deposits, which can be related to the mineralization. 5) Homogenization temperatures of fluid inclusions show 208° to 222℃. Remarkable features of the Taishu-type ore deposits are: 1) Mineralization was derived primarily in three stages, namely, Co-As, Fe-Zn-Pb, and rejuvenation stages. 2) All the ore deposits including 35 deposits in five areas are characterized by the higher temperature mineral assemblage, compared to the Shigekuma-type deposits. 3) These features are summarized with a serial mineralization which can be ascribed to an intimate association of the 12-million-year biotite granite. 4) The configuration of ores in the Co-As stage shows a district zoning with distance from granite, and that in the Fe-Zn-Pb stage clearly indicates a deposition-type zonality in the classification of facies zonality. The Shigekuma-type deposit is concluded as a product of coprecipitation with boiling and throttling of ore-forming solutions under far from equilibrium, because of the primarily disturbed structures of ore and gangue minerals and the remarkable variations of chemical compositions of vein-forming minerals.続きを見る
目次 Ⅰ.Introduction Ⅱ.Geology and ore deposits of Tsushima Ⅲ.Shigekuma-type ore deposits Ⅳ.Taishu-type mineralization Ⅴ.Considerations Ⅵ.Concluding remarks Acknowledgements References cited

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登録日 2015.11.11
更新日 2021.02.18

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